Refractory raw materials, method for production thereof and refractory using the material

ABSTRACT

Refractories obtained by molding a refractory raw material composition containing a refractory raw material including graphite grains having an average grain size of 500 nm or less or a refractory raw material including graphite grains obtained by graphitizing carbon black and a refractory filler. Or refractories containing carbonaceous grains (A) selected from carbon black or graphite grains obtained by graphitizing carbon black and having a DBP absorption (x) of 80 ml/100 g or more, carbonaceous grains (B) selected from carbon black and graphite grains obtained by graphitizing carbon black and having a DBP absorption (x) of less than 80 ml/100 g, and a refractory filler. Refractories excellent in corrosion resistance, oxidation resistance and thermal shock resistance, especially carbon-contained refractories having a low carbon content are thereby provided.

TECHNICAL FIELD

The present invention relates to a refractory raw material comprisinggraphite grains and a process for producing graphite grains which can beused therein. Further, it relates to a refractory raw materialcomposition containing the same. Still further, it relates to arefractory raw material composition comprising plural types of specificcarbonaceous grains selected from carbon black and graphite grainsobtained by graphitizing carbon black. Furthermore, it relates torefractories obtained by molding the refractory raw materialcomposition, especially, refractories excellent in corrosion resistance,oxidation resistance and thermal shock resistance and advantageous as alining of refining containers.

BACKGROUND ART

Since carbon has a property that it is hardly wetted with a melt such asa slag, carbon-contained refractories have an excellent durability.Accordingly, in recent years, they have been widely used as liningrefractories of various molten metal containers. For example, whenmagnesia is used as a refractory filler, an excellent durability isexhibited as lining refractories of molten metal containers because ofthe property provided by carbon and a corrosion resistance to meltprovided by magnesia.

However, as carbon-contained refractories have been increasingly used,elution of carbon of refractories in molten steel which is so-calledcarbon pickup has been problematic. Especially, in recent years,high-quality steel has been required more severely, and refractorieshaving a lower carbon content has been in high demand. Meanwhile, fromthe aspect of inhibition of heat dissipation from containers orenvironmental protection such as energy saving, the use of refractorieshaving a low thermal conductivity has been required. From thisstandpoint as well, refractories having a low carbon content has beendemanded.

As carbonaceous raw materials used in carbon-contained refractories,flake graphite, a pitch, a coke, mesocarbon and the like have been sofar mainly used. For obtaining refractories having a low carbon content,the mere reduction of the use amount of these carbonaceous raw materialshas involved a problem of the decrease in thermal shock resistance. Inorder to solve this problem, official gazette of JP-A-5-301772 proposesrefractories in which expanded graphite is used as a carbonaceous rawmaterial. Examples thereof describe a magnesia carbon brick obtained bykneading a refractory raw material composition comprising 95 parts byweight of sintered magnesia, 5 parts by weight of expanded graphite and3 parts by weight of a phenol resin, press-molding the composition andthen heat-treating the molded product at 300° C. for 10 hours. It isdescribed that a spalling resistance is improved in comparison to theuse of the same amount of flake graphite.

Official gazette of JP-A-11-322405 discloses carbon-containedrefractories having a low carbon content, characterized in that in a rawmaterial blend comprising a refractory raw material and a carbonaceousraw material containing carbon, a fixed carbon content of thecarbonaceous raw material is from 0.2 to 5% by weight per 100% by weightof a hot residue of the raw material blend and carbon black is used inat least a part of the carbonaceous raw material (claim 5). In theofficial gazette, it is explained that since carbon black has a smallgrain size of approximately 0.1 μm, a dispersibility in a refractorytexture is significantly high, surfaces of filler grains can be coatedwith fine carbon grains, and the contact of filler grains can be blockedeven at a high temperature over a long period of time to inhibitexcessive sintering. Examples describe refractories formed by molding araw material blend obtained by blending a refractory filler comprising50 parts by weight of magnesia and 50 parts by weight of alumina with2.5 parts by weight of a phenol resin, 1 part by weight of a pitch and 1part by weight of carbon black (thermal) and baking the molded productat from 120 to 400° C., indicating that the refractories are excellentin spalling resistance and resistance to oxidative damage.

Official gazette of JP-A-2000-86334 describes a brick for a slidingnozzle apparatus obtained by adding from 0.1 to 10% by weight, based onouter percentage, of carbon black having a specific surface area of 24m²/g or less to a blend comprising a refractory filler and a metal,further adding an organic binder, kneading the mixture, molding theresulting mixture and then heat-treating the molded product at atemperature of from 150 to 1,000° C. It is indicated that theincorporation of specific carbon black (thermal class or thermal blackclass) in a spherical form having a large grain size of from 80 to 500nm provides a good packing property and a dense brick texture todecrease a porosity and used carbon black itself is also excellent inoxidation resistance, whereby refractories excellent in oxidationresistance are obtained. Examples describe refractories obtained bymolding a blend comprising 97 parts by weight of alumina, 3 parts byweight of aluminum, 3 parts by weight of a phenol resin, 3 parts byweight of a silicon resin and 3 parts by weight of carbon black andheating the molded product at a temperature of 500° C. or less,indicating that the refractories are excellent in oxidation resistance.

Official gazette of JP-A-7-17773 describes monolithic refractories inwhich from 0.1 to 3% by weight of spherical carbon black having a largegrain size of from 0.02 to 0.50 μm and imperfect in structuredevelopment is added to a refractory filler. Further, official gazetteof JP-A-10-36177 describes a blast furnace taphole mud containing from 2to 15% by weight of carbon black having a DBP absorption of 100 ml/100 gor less and fixed amounts of a carbonaceous raw material, siliconcarbide, silicon nitride, a refractory raw material and acarbon-containing binder. Still further, official gazette ofJP-A-2000-192120 describes a taphole mud comprising a refractory filler,carbon black having a DBP absorption of from 15 to 80 ml/100 g, a pitchand a binder.

On the other hand, official gazette of JP-A-2000-273351 discloses aprocess for producing graphitized carbon black, which comprisesheat-treating a mixture containing carbon black and agraphitization-promoting substance at from 2,000 to 2,500° C. Thetemperature of approximately 2,800° C. so far required forgraphitization of carbon black can be reduced to from 2,000 to 2,500° C.by heating along with a graphitization-promoting substance made of anelement such as boron, silicon, aluminum or iron or its compound.

However, as described in JP-A-5-301772, the use of expanded graphite asa carbonaceous raw material can provide a good thermal shock resistanceeven in low-carbon refractories in which the use amount thereof isapproximately 5% by weight as compared to the use of flake graphite inthe same amount. Nevertheless, expanded graphite is a highly bulky rawmaterial. Accordingly, even when the use amount is as small asapproximately 5% by weight, a packing property of refractories isdecreased, and a corrosion resistance to melt is poor. Moreover, theoxidative loss of the carbonaceous raw material during use ofrefractories was also a serious problem.

Official gazettes' of JP-A-11-322405, JP-A-2000-86334, JP-A-7-17773,JP-A-10-36177 and JP-A-2000-192120 all describe examples of using carbonblack as a carbonaceous raw material. Although the employment of carbonblack was deemed to improve a spalling resistance, a corrosionresistance and an oxidation resistance were still insufficient. Further,carbon black used includes carbon black having a specific surface areaof less than 24 m²/g, spherical carbon black having a large grain sizeand imperfect in structure development and carbon black having a DBPabsorption of less than 100 ml/100 g or from 15 to 80 ml/100 g. That is,carbon black having a large grain size with a low DBP (dibutylphthalate) absorption is deemed to be rather preferable. The employmentof such a carbon black was, however, still insufficient to improve athermal shock resistance.

Further, for forming a dense texture or improving an oxidationresistance, a method in which a powder of a single substance ofaluminum, silicon, magnesium or the like or a powder of a compoundexcept an oxide, such as boron carbide or silicon carbide, was mainlyemployed. In this method, however, for obtaining sufficient effects,these additives had to be used in large amounts, which had, in manycases, an adverse effect on other characteristics consequently. For thisreason, there was no choice but to make a compromise at some level.

Official gazette of JP-A-2000-273351 describes a process in which carbonblack and a graphitization-promoting substance such as boron areheat-treated for graphitization. However, it is used in a carrier for acatalyst of a phosphoric acid-type fuel cell, and there is nothing todescribe or suggest that such a graphitized carbon black is useful as araw material of refractories.

The invention has been made for solving the foregoing problems. It is anobject of the invention to provide refractories excellent in corrosionresistance, oxidation resistance and thermal shock resistance,especially, carbon-contained refractories having a low carbon content.Such carbon-contained refractories having a low carbon content areuseful because carbon pickup in molten steel is reduced and heatdissipation from containers is decreased. Another object of theinvention is to provide a refractory raw material and a refractory rawmaterial composition for obtaining such refractories. Still anotherobject of the invention is to provide a process for producing graphitegrains which can be used in them. A further object of the invention isto provide a process for producing the refractory raw materialcomposition.

DISCLOSURE OF THE INVENTION

Refractories comprise grains having a wide variety of grain sizesranging from coarse grains having a size of approximately 5 mm to finegrains having a size of less than 1 μm, and an aggregate of fine grainswhich fill spaces of relatively large grains, the aggregate being calleda matrix, greatly influences a durability. In the matrix portion, alarge number of pores or voids are present, and influence a strength ofrefractories, a permeability of a melt such as a slag, relaxation of athermal shock and the like.

A grain size of a matrix in refractories is generally deemed to be lessthan 44 μm. Meanwhile, the present inventors have focused on the factthat a behavior of ultrafine grains having a size of less than 10 μm,further less than 1 μm, namely a size in the nanometer order has a greatinfluence. In carbon-contained refractories, a carbonaceous raw materialis, in most cases, used in the matrix portion. Studies have been made tocontrol properties of overall refractories by controlling a carbonaceousraw material in the nanometer order.

The inventors have conducted investigations upon focussing on thecontrol of the porous structure in controlling the carbonaceous rawmaterial in the nanometer order. The reduction of the amount of poresleads to the improvement in corrosion resistance, and the control of theform (specific surface area) of the pores or the fine division thereofcan contribute to providing an appropriate dynamic elastic modulus orimproving a thermal shock resistance. Thus, they have intended toimprove, by controlling the porous structure, the thermal shockresistance and further the corrosion resistance and the oxidationresistance.

As a carbonaceous raw material which is fine grains in the nanometerorder, carbon black is known. The porous structure can be controlled tosome extent by controlling the grain size. Nevertheless, when carbonblack is used as a matrix material, the corrosion resistance and theoxidation resistance are, in many cases, not necessarily sufficient.Consequently, assiduous studies have been made on a method for improvingthe corrosion resistance and the oxidation resistance of carbon blackitself with the grain size unchanged.

That is, the first invention is a refractory raw material comprisinggraphite grains having an average grain size of 500 nm or less. Sincegraphite is developed in crystal structure as compared to carbon black,it is a material which has a high oxidation-initiating temperature, isexcellent in oxidation resistance and also in corrosion resistance, andhas a high thermal conductivity. The use of fine graphite grains in thenanometer order can divide pores to control the porous structure andfurther improve the corrosion resistance and the oxidation resistance ofgrains per se, with the result that refractories excellent in thermalshock resistance, corrosion resistance and oxidation resistance areobtained.

Further, the first invention is a refractory raw material comprisinggraphite grains obtained by graphitizing carbon black. This is becausecarbon black is carbonaceous fine grains with the grain size in thenanometer order which can currently be procured easily and products withvarious trade names can easily be obtained according to purposes in viewof a grain size, an aggregation condition, a surface condition and thelike.

It is preferable that the graphite grains contain at least one elementselected from metals, boron and silicon. This is because formation of,so to speak, “composite graphite grains” in which graphite grainscontain such an element except carbon further increases theoxidation-initiating temperature of graphite grains per se, improves theoxidation resistance and the corrosion resistance and also improves theoxidation resistance and the corrosion resistance of refractoriesobtained by using the composite graphite grains as a raw material.

It is preferable that the graphite grains containing at least oneelement selected from metals, boron and silicon are obtained by heatingcarbon black and a simple substance of at least one element selectedfrom metals, boron and silicon or a compound containing the element. Itis more preferable that the graphite grains are obtained by heatingcarbon black and a simple substance of at least one element selectedfrom metals, boron and silicon.

Further, the invention is a refractory raw material compositioncomprising a refractory filler and the graphite grains. At this time, arefractory raw material composition comprising 100 parts by weight ofthe refractory filler and from 0.1 to 10 parts by weight of the graphitegrains is preferable.

Still further, the invention is a process for producing graphite grainscontaining at least one element selected from metals, boron and silicon,characterized by heating carbon black and an alcoholate of at least oneelement selected from metals, boron and silicon. This is because when anelement which is dangerous in the form of a simple substance because ofeasy explosion is formed into an alcoholate, it becomes easy to handle,and a risk of dust explosion or the like is reduced. At this time, whenthe graphite grains produced by the process of the invention are used asa refractory raw material as mentioned above, the problems of theinvention are, needless to say, resolved. Moreover, the process of theinvention is useful because it can also be used for other purposes. Thispoint is also the same with the following two processes.

Moreover, the invention is a process for producing graphite grainscontaining at least one element selected from metals, boron and silicon,characterized by heating carbon black, an oxide of at least one elementselected from metals, boron and silicon and a metal reducing the oxide.With such a combination, the element constituting the oxide can easilybe reduced and contained in graphite.

In addition, the invention is a process for producing graphite grains,which comprises heating carbon black and a simple substance of at leastone element selected from metals, boron and silicon or a compoundcontaining the element, and further oxidizing the resulting graphitegrains. Consequently, the more improved oxidation resistance isprovided.

Meanwhile, a porous structure can be controlled to some extent bycontrolling the grain size of carbon black. As stated above, however,with the mere use of carbon black having a low DBP absorption, thethermal shock resistance is still insufficient. On the other hand, withthe mere use of carbon black having a high DBP absorption, the oxidationresistance and the corrosion resistance are insufficient as will belater described in Comparative Examples. The second invention has beenattained as a result of assiduous studies to solve such problems.

That is, the second invention is a refractory raw material compositioncomprising carbonaceous grains (A) selected from carbon black andgraphite grains obtained by graphitizing carbon black and having a DBPabsorption (x) of 80 ml/100 g or more, carbonaceous grains (B) selectedfrom carbon black and graphite grains obtained by graphitizing carbonblack and having a DBP absorption (x) of less than 80 ml/100 g, and arefractory filler. The DBP absorption (x) here referred to is a value(ml/100 g) measured by a method defined in A method of “DBP Absorption”in item 9 of JIS K6217.

The use of carbonaceous grains (A) selected from carbon black andgraphite grains obtained by graphitizing carbon black and having a highDBP absorption can form quite a fine porous structure in a matrix ofrefractories, provide an appropriate dynamic elastic modulus and improvea thermal shock resistance. The dynamic elastic modulus is an index ofthe thermal shock resistance. The lower the dynamic elastic modulus, thebetter the thermal shock resistance. Refractories excellent in thermalshock resistance can inhibit spalling damage when actually used.However, the mere use of the carbonaceous grains (A) is insufficient inoxidation resistance and corrosion resistance. The combined use ofcarbonaceous grains (B) having a low DBP absorption can improve thispoint, with the result that refractories excellent in thermal shockresistance, corrosion resistance and oxidation resistance are obtained.

At this time, it is preferable that the total weight of the carbonaceousgrains (A) and the carbonaceous grains (B) is from 0.1 to 10 parts byweight per 100 parts by weight of the refractory filler and the weightratio (A/B) of the carbonaceous grains (A) to the carbonaceous grains(B) is from 1/99 to 99/1. It is also preferable that the average primarygrain size of the carbonaceous grains (A) is from 10 to 50 nm and theaverage primary grain size of the carbonaceous grains (B) is from 50 to500 nm.

It is preferable that the ratio (x/y) of the DBP absorption (x) of thecarbonaceous grains (A) to the DBP absorption (y) of the compressedsample of the carbonaceous grains (A) is 1.15 or more. The DBPabsorption (y) of the compressed sample here referred to is a value(ml/100 g) measured by the method defined in “DBP Absorption ofCompressed Sample” in item 10 of JIS K6217. Carbonaceous grains whoseDBP absorption after compression is decreased indicate that thestructure of carbonaceous grains is changed by a compression procedure.Specifically, it suggests formation of an aggregate in which primarygrains are aggregated. The use of such carbonaceous grains (A) canprovide the excellent thermal shock resistance.

In view of improving the oxidation resistance and the corrosionresistance, it is preferable that at least the carbonaceous grains (A)or the carbonaceous grains (B) are graphite grains obtained bygraphitizing carbon black, and it is more preferable that both of thecarbonaceous grains (A) and the carbonaceous grains (B) are graphitegrains obtained by graphitizing carbon black. The use of graphite grainsimproves the corrosion resistance and the oxidation resistance of grainsper se, with the result that refractories excellent in thermal shockresistance, corrosion resistance and oxidation resistance are obtained.

In view of improving the oxidation resistance and the corrosionresistance, it is more preferable that at least the carbonaceous grains(A) or the carbonaceous grains (B) are graphite grains obtained bygraphitizing carbon black and the graphite grains contain at least oneelement selected from metals, boron and silicon, and it is mostpreferable that both of the carbonaceous grains (A) and the carbonaceousgrains (B) are graphite grains obtained by graphitizing carbon black andthe graphite grains contain at least one element selected from metals,boron and silicon. This is because formation of, so to speak, “compositegraphite grains” in which graphite grains contain such an element exceptcarbon further increases the oxidation-initiating temperature ofgraphite grains per se, improves the oxidation resistance and thecorrosion resistance and also improves the oxidation resistance and thecorrosion resistance of refractories obtained by using the compositegraphite grains as a raw material.

In producing the refractory raw material composition, it is preferableto previously disperse the carbonaceous grains (A) in an organic binderand then mix the dispersion with the other raw materials because thedispersibility of the carbonaceous grains (A) in the matrix can beimproved and consequently refractories improved in thermal shockresistance, oxidation resistance and corrosion resistance can beobtained.

In the refractory raw material composition in the first and secondinventions, a refractory raw material composition in which a refractoryfiller comprises magnesia is preferable in consideration of usefulapplications of refractories having a low carbon content. Further, theinvention is a refractory which is obtained by molding the refractoryraw material composition.

The Invention is Described in Detail Below.

The first invention is a refractory raw material comprising graphitegrains having an average grain size of 500 nm or less. It is hereimportant that the average grain size is 500 nm or less, and the use ofthe graphite grains having such a fine grain size can provide a fineporous structure in the matrix of refractories. Flake graphite andexpanded graphite used so far as a refractory raw material both had agrain size greatly exceeding 1 μm and could not develop a fine porousstructure in a matrix. Such a porous structure can be realized uponusing the fine graphite grains of the invention.

The average grain size is preferably 200 nm or less, more preferably 100nm or less. Further, the average grain size is usually 5 nm or more,preferably 10 nm or more. When the average grain size exceeds 500 nm, afine porous structure cannot be provided. When it is less than 5 nm,grains are difficult to handle. The average grain size here referred toindicates a number average grain size of primary grains of graphitegrains. Accordingly, in case of, for example, grains having a structurethat plural primary grains are aggregated, a number average grain sizeis calculated on condition that plural primary grains constituting thesame are contained. Such a grain size can be measured by observationwith an electron microscope.

A process for producing graphite grains is not particularly limited, andgraphite having a larger grain size may be pulverized to the foregoinggrain size mechanically or electrically. However, since it is difficultto pulverize grains into quite fine grains having a grain size of 500 nmor less, a process in which carbonaceous grains originally having agrain size of 500 nm or less are graphitized is preferable.

Moreover, the invention is a refractory raw material comprising graphitegrains obtained by graphitizing carbon black. Carbon black iscarbonaceous fine grains with a grain size in the nanometer order whichcan easily be procured, and grains with various trade names can easilybe obtained according to purposes in view of a grain size, anaggregation condition, a surface condition and the like. It is alreadyknown that carbon black itself is used as a refractory raw material asdescribed in column Prior Art. However, carbon black was insufficient incorrosion resistance and oxidation resistance. By graphitizing it, thecrystal structure is developed, and a material which is high inoxidation-initiating temperature, excellent in oxidation resistance andalso in corrosion resistance and high in thermal conductivity can beformed.

Carbon black as a raw material is not particularly limited. Carbon blackcomprising primary grains having a size of 500 nm or less is preferablyused. Specifically, any of furnace black, channel black, acetyleneblack, thermal black, lamp black, Ketjen black and the like can be used.

Preferable examples thereof include various carbon blacks such as firstextruding furnace black (FEF), super abrasion furnace black (SAF), highabrasion furnace black (HAF), fine thermal black (FT), medium thermalblack (MT), semi-reinforcing furnace black (SRF) and general-purposefurnace black (GPF). At this time, plural types of carbon blacks may beblended and used as a raw material.

Although a method for graphitizing carbon black is not particularlylimited, it can be graphitized by heating at a high temperature in aninert atmosphere. Usually, carbon black can be graphitized by heating ata temperature of more than 2,000° C.

By the graphitization, a peak ascribable to a crystal structure isobserved in the X-ray diffraction measurement. As the graphitizationproceeds, lattice spacing is shortened. A 002 diffraction line ofgraphite shifts to a wide-angle region as the graphitization proceeds,and a diffraction angle 2θ of this diffraction line corresponds to thelattice spacing (average spacing). In the invention, it is preferable touse graphite of which the lattice spacing d is 3.47 Å or less. When thelattice spacing exceeds 3.47 Å, the graphitization is insufficient, andthe thermal shock resistance, the oxidation resistance and the corrosionresistance might be insufficient.

With respect to the graphite grains, it is preferable that the graphitegrains contain at least one element selected from metals, boron andsilicon. This is because formation of, so to speak, “composite graphitegrains” in which graphite grains contain such an element except carbonfurther increases the oxidation-initiating temperature of graphitegrains per se, improves the oxidation resistance and the corrosionresistance and also improves the oxidation resistance and the corrosionresistance of refractories obtained by using the composite graphitegrains as a raw material.

Specific examples of at least one element which is contained in thegraphite grains and selected from metals, boron and silicon here includeelements such as magnesium, aluminum, calcium, titanium, chromium,cobalt, nickel, yttrium, zirconium, niobium, tantalum, molybdenum,tungsten, boron and silicon. Of these, for improving the oxidationresistance and the corrosion resistance of refractories, boron,titanium, silicon, zirconium and nickel are preferable, and boron andtitanium are most preferable.

The way in which each element is present in the graphite grains is notparticularly limited, and it may be contained within the grains or so asto cover surfaces of grains. Further, each element can be contained asan oxide, a nitride, a borate or a carbide thereof. It is preferablycontained as a compound such as an oxide, a nitride, a borate or acarbide. It is more preferably contained as a carbide or an oxide. B₄Cor TiC is shown as a carbide, and Al₂O₃ is shown as an oxide.

The carbide is properly contained in the graphite grains in a form boundto a carbon atom constituting graphite. It is, however, undesirable thatthe total amount of the graphite grains is contained as the carbidebecause properties as graphite cannot be exhibited. Thus, it isnecessary that the graphite grains have the crystal structure ofgraphite. The condition of such graphite grains can be analyzed by X-raydiffraction. For example, besides the peak corresponding to the crystalof graphite, a peak corresponding to the crystal of the compound such asTiC or B₄C is observed.

A process in which at least one element selected from metals, boron andsilicon is contained in the graphite grains is not particularly limited.It is preferable that the graphite grains are obtained by heating carbonblack and a simple substance of at least one element selected frommetals, carbon and silicon or a compound containing the element. By theheating, the graphitization proceeds, and at the same time, the elementis contained in the graphite structure.

At this time, it is more preferable that the graphite grains areobtained by heating carbon black and a simple substance of at least oneelement selected from metals, boron and silicon. This is because byheating with a simple substance of an element, the reaction can proceedwith heat generated during formation of a carbide through burningsynthesis. Specifically, it is preferable to conduct the heating withaluminum, calcium, titanium, zirconium, boron or silicon. This isbecause the synthesis is enabled by a self-burning synthesis method withthis reaction heat. Since the reaction heat of its own can be utilized,the temperature inside the furnace can be reduced as compared to thecase of graphitizing carbon black alone. Maintaining the furnacetemperature over 2,000° C. is quite problematic in view of an apparatusand a cost. Thus, the very point is important.

For example, a reaction formula of the burning synthesis of boron andcarbon and a reaction formula of the burning synthesis of titanium andcarbon are as follows.4B+xC→B₄C+(x−1)CTi+xC→TiC+(x−1)C

Both of these reactions are exothermic reactions which allowself-burning synthesis.

As a process in which at least one element selected from metals, boronand silicon is contained in the graphite grains, it is also preferableto heat carbon black and an alcoholate of at least one element selectedfrom metals, boron and silicon because of the use of heat generated byburning synthesis. This is because when an element which is dangerous inthe form of a simple substance because of easy explosion is formed intoan alcoholate, it becomes easy to handle and a risk of dust explosion orthe like is reduced.

The alcoholate here referred to is a compound in which hydrogen of ahydroxyl group of an alcohol is substituted with at least one elementselected from metals, boron and silicon, as represented by M(OR)_(n).Here, as M, a monovalent to tetravalent element, preferably a divalentto tetravalent element is used. Preferable examples of the elementinclude magnesium, aluminum, titanium, zirconium, boron and silicon, ncorresponds to a valence number of an element M, and it is an integer offrom 1 to 4, preferably an integer of from 2 to 4. Further, R is notparticularly limited so long as it is an organic group. It is preferablyan alkyl group having from 1 to 10 carbon atoms, and examples thereofinclude a methyl group, an ethyl group, a propyl group, an isopropylgroup, an n-butyl group and the like. These alcoholates may be usedeither singly or in combination. Moreover, it is also possible to use asimple substance or an oxide of an element and an alcoholate thereof incombination.

As a process in which at least one element selected from metals, boronand silicon is contained in the graphite grains, it is preferable toheat carbon black, an oxide of at least one element selected frommetals, boron and silicon and a metal reducing the oxide because heatgenerated by burning synthesis can be utilized. By such a combination,it is possible that a metal reduces an oxide and an element constitutingan oxide is contained in graphite. For example, when carbon black,aluminum and boron oxide are heated, boron oxide is first reduced withaluminum to form a simple substance of boron which is reacted withcarbon black to obtain boron carbide. This is shown by the followingchemical formula.4Al+2B₂O₃+xC→2Al₂O₃+B₄C+(x−1)C

Further, a chemical formula in case of reacting carbon black, aluminumand titanium oxide is as follows.4Al+3TiO₂+xC→2Al₂O₃+3TiC+(x−3)C

These reactions are also exothermic reactions. Burning synthesis ispossible, and graphitization can be conducted even though a temperatureinside a furnace is not so high.

Moreover, it is also preferable that carbon black and a simple substanceof at least one element selected from metals, boron and silicon or acompound containing the element are heated and the resulting graphitegrains are further oxidized. By the oxidization, coatings of the oxidecan be formed mainly on surfaces of the graphite grains, and thegraphite grains are much better in oxidation resistance.

The oxidation method is not particularly limited. For example, a methodin which grains are treated with a gas having a high temperature capableof oxidation is mentioned. Specifically, a so-called hot gas method inwhich a hot gas generated by burning air and a fuel is reacted withgraphite grains for a prescribed period of time can be mentioned. Atthis time, when the time of contact with the gas is too long, theoverall graphite is oxidized. It is thus necessary to determine suchconditions that only a part thereof can be oxidized.

Needless to say, the graphite grains produced by the foregoing processresolve the problems of the invention when used as a refractory rawmaterial as stated above. The process of the invention is useful becauseit can also be used for other purposes.

The above-obtained graphite grains are blended with the other ingredientto form the refractory raw material composition of the invention.Specifically, a refractory raw material composition comprising arefractory filler and the graphite grains is formed.

The second invention is a refractory raw material composition comprisingboth carbonaceous grains (A) selected from carbon black and graphitegrains obtained by graphitizing carbon black and having a DBP absorption(x) of 80 ml/100 g or more and carbonaceous grains (B) selected fromcarbon black and graphite grains obtained by graphitizing carbon blackand having a DBP absorption (x) of less than 80 ml/100 g.

Carbon black is carbonaceous fine grains with the grain size in thenanometer order which can currently be procured easily, and productswith various trade names can easily be obtained according to purposes inview of a grain size, an aggregation condition, a surface condition andthe like. Specific examples thereof include furnace black, channelblack, acetylene black, thermal black, lamp black, Ketjen black and thelike. Carbon black is usually carbonaceous grains having an averageprimary grain size of 500 nm or less, and graphite grains obtained bygraphitizing the same have approximately the same average grain size.The use of the carbonaceous grains having such a fine grain size canmake fine the porous structure in the matrix of refractories. Flakegraphite and expanded graphite widely used so far as a refractory rawmaterial had both an average grain size greatly exceeding 1 μm and couldnot provide a fine porous structure in a matrix. However, the use of thefine carbonaceous grains can realize the fine porous structure.

By using the carbonaceous grains (A) having the DBP absorption (x) of 80ml/100 g or more allows the formation of the quite fine porous structurein the matrix of refractories, and can reduce the dynamic elasticmodulus to improve the thermal shock resistance. The DBP absorption (x)here referred to is a value measured by a method defined in A method of“DBP Absorption” in item 9 of JIS K6217. The DBP absorption of thecarbonaceous grains (A) is preferably 90 ml/100 g or more, morepreferably 100 ml/100 g or more. Further, the DBP absorption of thecarbonaceous grains (A) is usually 1,000 ml/100 g or less.

The average primary grain size of such carbonaceous grains (A) ispreferably from 10 to 50 nm. When it is less than 50 nm, quite a fineporous structure is easily formed in the matrix of refractories. It ismore preferably 45 nm or less. In view of the easy handling, theoxidation resistance and the corrosion resistance, it is preferably 15nm or more, more preferably 20 nm or more. The average primary grainsize can be measured by observation with an electron microscope. At thistime, in case of grains having a structure that plural primary grainsare aggregated, calculation is conducted on condition that pluralprimary grains constituting the same are contained.

Moreover, in the carbonaceous grains (A), it is preferable that theratio (x/y) of the DBP absorption (x) to the DBP absorption (y) of thecompressed sample is 1.15 or more. The DBP absorption (y) of thecompressed sample here referred to is a value measured by a methoddefined in “DBP Absorption of Compressed Sample” in item 10 of JISK6217, and it is a DBP absorption after a compression procedure at apressure of 165 MPa is repeated four times.

Carbon black includes carbon black comprising spherical single grainsand carbon black in the form of an aggregate in which primary grains aremutually aggregated. FIG. 1 is a schematic view of carbon black in theform of an aggregate. As the carbonaceous grains (A) of the invention,grains in the form of an aggregate are preferably used. That the ratio(x/y) of the DBP absorption (x) to the DBP absorption (y) of thecompressed sample is 1.15 or more means that carbon black causes morethan a certain structural change by the compression procedure. Morespecifically, it means that a rate of an aggregate to be deformed ordestroyed in the compression is high and the DBP absorption is thereforedecreased by more than a certain rate. The ratio (x/y) is preferably 1.2or more, more preferably 1.3 or more. The ratio (x/y) is usually 2 orless.

That the ratio (x/y) is high means that an aggregate tends to bedeformed or destroyed when the grains are used as refractories andundergo stress thermally or mechanically. That is, when stress occurs ina matrix in using the grains as refractories, an energy can be absorbedby deformation or destruction of an aggregate to relax the stress. Thatis, when crack that occurs and proceeds in a matrix reaches carbon blackin the form of an aggregate, it blocks the progress thereof, showingthat an excellent thermal shock resistance as refractories is provided.

Further, for example, aggregates linearly connected can also workthemselves as a reinforcement of a matrix and heat conduction throughaggregates is good. In this respect as well, the thermal shockresistance is improved. Besides, in carbon black having a relativelysmall average primary grain size, such aggregates are often formed.Therefore, formation of fine pores in the matrix is also attained at thesame time. That is, it is possible to control quite fine pores in thenanometer order, with the result that refractories excellent in thermalshock resistance are provided. The effects provided by formation of theaggregates are given to not only the use of carbon black but also theuse of graphitized carbon black as the carbonaceous grains (A).

Carbon black available as the carbonaceous grains (A) is notparticularly limited. Specifically, preferable examples thereof includefirst extruding furnace black (FEF), super abrasion furnace black (SAF)and high abrasion furnace black (HAF). When graphite grains obtained bygraphitizing carbon black are used as the carbonaceous grains (A),preferable graphite grains can be produced using this carbon black as araw material. Further, the carbonaceous grains (A) may be a mixture ofplural types of the carbonaceous grains (A).

When the foregoing carbonaceous grains (A) alone are incorporated in therefractory raw material composition, the thermal shock resistance can beimproved by formation of fine pores, but the oxidation resistance andthe corrosion resistance tend to decrease. For this reason, theinvention uses the carbonaceous grains (A) and the carbonaceous grains(B) in combination.

When the carbonaceous grains (B) selected from carbon black and graphitegrains obtained by graphitizing carbon black and having the DBPabsorption of less than 80 ml/100 g are used in combination with thecarbonaceous grains (A), the packing density of refractories can beincreased, and the oxidation resistance and the corrosion resistance canbe improved. The DBP absorption of the carbonaceous grains (B) ispreferably 60 ml/1000 g or less, more preferably 40 ml/100 g or less.Further, the DBP absorption of the carbonaceous grains (B) is usually 10ml/100 g or more.

The average primary grain size of such carbonaceous grains (B) ispreferably from 50 to 500 nm. When it is 50 nm or more, the packingproperty is good in the matrix of refractories, and the oxidationresistance and the corrosion resistance are improved. It is morepreferably 60 nm or more. When it exceeds 500 nm, the size of the porein the matrix is too large, and the thermal shock resistance is notablydecreased. It is more preferably 200 nm or less, further preferably 100nm or less.

Such carbonaceous grains (B), unlike the carbonaceous grains (A), lessform an aggregate in which primary grains are mutually aggregated, andmost of the grains comprise single spheres, which is preferable in viewof the packing property. Accordingly, the carbonaceous grains in whichthe ratio (x/y) of the DBP absorption (x) to the DBP absorption (y) ofthe compressed sample is less than 1.15 are preferably used. The ratio(x/y) is more preferably 1.1 or less, further preferably 1.05 or less.When carbon black comprises independent spheres which are not mutuallyaggregated and the structure is never destroyed by the compression, theratio (x/y) is theoretically 1. Actually, however, it might include somemeasurement error. The ratio (x/y) as a measured value is usually 0.9 ormore. By using in combination the carbonaceous grains (A) mainly havingthe aggregate structure in which the grains are highly aggregated andthe carbonaceous grains (B) mainly containing the single spheres, quitea fine porous structure can be developed while securing the high packingrate.

Carbon black available as the carbonaceous grains (B) is notparticularly limited. Specifically, preferable examples thereof includefine thermal black (FT), medium thermal black (MT), semi-reinforcingfurnace black (SRF) and general-purpose furnace black (GPF). Whengraphite grains obtained by graphitizing carbon black are used as thecarbonaceous grains (B), preferable graphite grains can be producedusing this carbon black as a raw material. Further, the carbonaceousgrains (B) may be a mixture of plural types of the carbonaceous grains(B).

The weight ratio (A/B) of the carbonaceous grains (A) to thecarbonaceous grains (B) is preferably 1/99 to 99/1. When the weightratio (A/B) is less than 1/99, the thermal shock resistance might beinsufficient. When it exceeds 99/1, the corrosion resistance or theoxidation resistance might be insufficient. The weight ratio (A/B) ismore preferably 5/95 or more, further preferably 10/90 or more. Theweight ratio (A/B) is more preferably 90/10 or less, further preferably70/30 or less.

In view of the oxidation resistance and the corrosion resistance, it ispreferable that at least the carbonaceous grains (A) or the carbonaceousgrains (B) are graphite grains obtained by graphitizing carbon black.The graphite grains areas described in the first invention.Ingraphitizing carbon black, the DBP absorption, the DBP absorption ofthe compressed sample and the average primary grain size are usually notchanged much.

In the invention, it is especially preferable that the carbonaceousgrains (A) are graphite grains obtained by graphitizing carbon black.This is because carbon black used as the carbonaceous grains (A) isinferior in corrosion resistance and oxidation resistance to carbonblack used as the carbonaceous grains (B) and this defect is offset bythe graphitization. A preferable embodiment is that the carbonaceousgrains (A) are graphite grains obtained by graphitizing carbon black andthe carbonaceous grains (B) are carbon black. In this case, the graphitegrains obtained by graphitizing carbon black are more expensive thanusual carbon black. Consequently, it is preferable from the economicalstandpoint that the mixing amount of the carbonaceous grains (A) issmaller than the mixing amount of the carbonaceous grains (B).

A more preferable embodiment of the invention is that both of thecarbonaceous grains (A) and the carbonaceous grains (B) are graphitegrains obtained by graphitizing carbon black. In this case, thecarbonaceous grains (A) and the carbonaceous grains (B) used are bothsuperior in corrosion resistance and oxidation resistance to carbonblack. As a result, the corrosion resistance and the oxidationresistance of refractories are more improved.

Further, it is preferable that at least the carbonaceous grains (A) orthe carbonaceous grains (B) are graphite grains obtained by graphitizingcarbon black and the graphite grains contain at least one elementselected from metals, boron and silicon. The oxidation resistance andthe corrosion resistance can be improved by not only graphitizing carbonblack but also containing at least one element selected from metals,boron and silicon. The “composite graphite grains” containing thegraphite grains and the element except carbon are as described in thefirst invention.

At this time, it is preferable that the carbonaceous grains (A) aregraphite grains obtained by graphitizing carbon black and the graphitegrains contain at least one element selected from metals, boron andsilicon. This is because the carbonaceous grains (A) are inferior incorrosion resistance and oxidation resistance to the carbonaceous grains(B) and this defect can thereby be offset.

At this time, a preferable embodiment is that the carbonaceous grains(A) are graphite grains obtained by graphitizing carbon black, thegraphite grains contain at least one element selected from metals, boronand silicon and the carbonaceous grains (B) are ungraphitized carbonblack. In this case, since the graphite grains obtained by graphitizingcarbon black and containing at least one element selected from metals,boron and silicon are more expensive than usual carbon black, it ispreferable that the amount of the carbonaceous grains (A) is smallerthan the amount of the carbonaceous grains (B).

Another preferable embodiment is that the carbonaceous grains (A) aregraphite grains obtained by graphitizing carbon black, the graphitegrains contain at least one element selected from metals, boron andsilicon, the carbonaceous grains (B) are graphite grains obtained bygraphitizing carbon black and the graphite grains are free from theelement. This is because upon comparing these graphite grains, thecarbonaceous grains (A) are inferior in corrosion resistance andoxidation resistance to the carbonaceous grains (B) and this defect canbe offset by containing at least one element selected from metals, boronand silicon.

The most preferable embodiment of the invention is that both of thecarbonaceous grains (A) and the carbonaceous grains (B) are graphitegrains obtained by graphitizing carbon black and the graphite grainscontain at least one element selected from metals, boron and silicon. Inthis case, both of the carbonaceous grains (A) and the carbonaceousgrains (B) are excellent in corrosion resistance and oxidationresistance in particular, with the result that the corrosion resistanceand the oxidation resistance of refractories become quite excellent.

Another ingredient is incorporated in the carbonaceous grains (A) andthe carbonaceous grains (B) described above to form the refractory rawmaterial composition of the invention. Specifically, the refractory rawmaterial composition comprising the carbonaceous grains (A), thecarbonaceous grains (B) and the refractory filler is provided.

In the refractory raw material composition of the first invention, therefractory filler mixed with the graphite grains is not particularlylimited. Further, in the refractory raw material composition of thesecond invention, the refractory filler mixed with the carbonaceousgrains (A) and the carbonaceous grains (B) is not particularly limited.In these refractory raw material compositions, various refractoryfillers can be used on the basis of the purpose and the requiredproperties as refractories. Refractory oxides such as magnesia, calcia,alumina, spinel and zirconia, carbides such as silicon carbide and boroncarbide, borates such as calcium borate and chromium borate, andnitrates can be used as the refractory filler. Of these, magnesia,alumina and spinel are preferable in consideration of usefulness of thelow carbon content, and magnesia is most preferable. As magnesia, anelectro-fused or sintered magnesia clinker is mentioned. Theserefractory fillers are incorporated after adjusting the grain size.

In the refractory raw material composition of the first invention, it ispreferable that the mixing amount of the graphite grains is from 0.1 to10 parts by weight per 100 parts by weight of the refractory filler. Inthe refractory raw material composition of the second invention, it isalso preferable that the total amount of the carbonaceous grains (A) andthe carbonaceous grains (B) is from 0.1 to 10 parts by weight per 100parts by weight of the refractory filler. When the total amount of thesegrains is less than 0.1 part by weight, the effects provided by theaddition of these grains are little found, and the thermal shockresistance is, in many cases, insufficient. It is preferably 0.5 part byweight or more. Meanwhile, when the total amount of these particlesexceeds 10 parts by weight, the carbon pickup drastically occurs, theheat dissipation from containers also heavily occur, and the corrosionresistance is decreased. It is preferably 5% by weight or less.

Moreover, as the binder used in the refractory raw material compositionof the invention, an ordinary organic binder or inorganic binder can beused. As a highly refractory binder, the use of an organic binder mademainly of a phenol resin, a pitch or the like is preferable. In view ofa wettability of a refractory raw material or a high content of residualcarbon, a binder made mainly of a phenol resin is more preferable. Anorganic binder may contain a solvent, and an appropriate viscosity canbe provided in blending by containing a solvent. The content of such anorganic binder is not particularly limited. It is preferably from 0.5 to10 parts by weight, more preferably from 1 to 5 parts by weight per 100parts by weight of the refractory filler.

In the refractory raw material composition of the second invention, aprocess in which the binder is incorporated in the refractory rawmaterial composition of the invention is not particularly limited. Aprocess in which the carbonaceous grains (A) are previously dispersed inthe organic binder and the dispersion is then mixed with the other rawmaterials is preferable. Since the carbonaceous grains (A) have a smallaverage primary grain size and form an aggregate in many cases, anaggregate tends to be formed in blending with other raw materials.However, after the grains are dispersed in the organic binder by beingpreviously stirred with the organic binder, the dispersion is blendedwith the other raw materials such as the organic filler and the like,whereby the carbonaceous grains (A) can be dispersed well in the matrix.

At this time, it is possible that both of the carbonaceous grains (A)and the carbonaceous grains (B) are previously dispersed in the organicbinder and the dispersion is then blended with the other raw material.In this case, however, the amount of the organic binder based on thetotal weight of the carbonaceous grains (A) and the carbonaceous grains(B) is small in many cases, and they might not be dispersed wellpreviously. Accordingly, it is preferable that only the carbonaceousgrains (A) difficult to disperse are previously dispersed in the organicbinder and the dispersion is then blended with the carbonaceous grains(B) and the refractory filler, because the carbonaceous grains (A) canbe dispersed well in the matrix.

In the refractory raw material composition of the first invention, thegraphite grains are used as the carbonaceous raw material. Further, inthe refractory raw material composition of the second invention, thecarbonaceous grains (A) and the carbonaceous grains (B) are used as thecarbonaceous raw material. In both of the refractory raw materialcompositions, another carbonaceous raw material may further be used incombination. For example, another graphite ingredient such as flakegraphite or expanded graphite may be used in combination, or a pitch, acoke or the like may be used in combination.

The refractory raw material composition of the invention may containingredients other than the foregoing unless the gist of the invention isimpaired. For example, metallic powders such as aluminum and magnesium,alloy powders, silicon powders and the like may be contained therein.Further, in kneading, an appropriate amount of water or a solvent may beadded.

The refractory of the invention is obtained by kneading thethus-obtained refractory raw material composition, molding thecomposition, and as required, heating the molded product. Here, in theheating, the product may be baked at a high temperature. However, incase of magnesia, the product is only baked at a temperature of,usually, less than 400° C.

A so-called monolithic refractory is included in the refractory rawmaterial composition of the invention when the refractory is monolithic.When the monolithic refractory comes to have a certain form, it isincluded in the molded refractory of the invention. For example, even aproduct sprayed on a furnace wall, it is included in the moldedrefractory of the invention since it has a certain shape.

Since the thus-obtained refractory is excellent in corrosion resistance,oxidation resistance and thermal shock resistance, it is quite useful asa furnace material for obtaining a high-quality metallurgical product.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is illustrated below by referring to Examples.

In Examples, analysis and evaluation were performed by various methodsto follow.

(1) Method for Observing an Average Primary Grain Size

A sample was photographed with 100,000 × magnification using atransmission electron microscope. From the resulting photograph, anumber average value of a size was obtained. At this time, when grainsof the sample are aggregated, these were considered to be separategrains, and a value was obtained as an average primary grain size.

(2) Method for Calculating Graphite Lattice Spacing

A graphite powder to be intended was measured using a powder X-raydiffractometer. A measurement wavelength λ is 1.5418 Å, a wavelength ofKα rays of copper. Of crystal peaks obtained by the X-ray diffractionmeasurement, a large peak of which the value of 2θ is present near 26°is a peak corresponding to a 002 surface of graphite. From this, thelattice spacing d(Å) of graphite was calculated using the followingformula.d=λ/2 sin θ

(3) Apparent Porosity and Bulk Specific Gravity After Treatment at1,400° C.

A sample cut to 50×50×50 mm was embedded in a coke within an electricfurnace, and heat-treated in an atmosphere of carbon monoxide at 1,400°C. for 5 hours. The treated sample was allowed to cool to roomtemperature, and an apparent porosity and a bulk specific gravity werethen measured according to JIS R2205.

(4) Dynamic Elastic Modulus

A sample of 110×40×40 mm was embedded in a coke within an electricfurnace, and heat-treated in an atmosphere of carbon monoxide at 1,000°C. or 1,400° C. for 5 hours. The treated sample was allowed to cool toroom temperature, and an ultrasonic wave propagation time was measuredusing an ultrasony scope. A dynamic elastic modulus E was obtained onthe basis of the following formula.E=(L/t)²·ρ

wherein L is an ultrasonic wave propagation distance (length of asample) (mm), t is an ultrasonic wave propagation time (μsec), and ρ isa bulk specific gravity of a sample.

(5) Oxidation Resistance Test

A sample of 40×40×40 mm was kept in an electric oven (ambientatmosphere) at 1,400° C. for 10 hours, and then cut. Thicknesses ofdecarbonized layers of three surfaces except a lower surface weremeasured at the cut face, and an average value thereof was calculated.

(6) Corrosion Resistance Test

A sample of 110×60×40 mm was installed on a rotary corrosion tester, anda test was conducted in which a step of keeping the sample in a slagwith a basicity (CaO/SiO₂)=1 held at from 1,700 to 1,750° C. wasrepeated five times. A wear size was measured in a cut surface after thetest.

SYNTHESIS EXAMPLE 1 Synthesis of Graphite Grains A (Carbonaceous Grainsb)

“Niteron #10 Kai” made by Nippon Steel Chemical Carbon Co., Ltd. wasused as a carbon black raw material. This carbon black is carbon blackof the type called first extruding furnace black (FEF) in which theaverage primary grain size is 41 nm, the DBP absorption (x) is 126ml/100 g and the DBP absorption (y) of the compressed sample is 89ml/100 g, and it is carbonaceous grains a used in this Example. Thiscarbon black was graphitized by heat treatment in a carbon furnace(FVS-200/200/200, FRET-50, manufactured by Fuji Electronics IndustryCo., Ltd. ) in an argon gas atmosphere at 2,100° C. for 3 hours toobtain graphite grains A (carbonaceous grains b). When the resultinggrains were subjected to the X-ray diffraction measurement, a peakascribable to a graphite structure was observed, and it was found thatgraphite grains were formed. Lattice spacing calculated from adiffraction line corresponding to 002 spacing of graphite was 3.40 Å.The average primary grain size of the grains was 38 nm, the DBPabsorption (x) thereof was 118 ml/100 g, and the DBP absorption (y) ofthe compressed sample thereof was 85 ml/100 g.

SYNTHESIS EXAMPLE 2 Synthesis of Graphite Grains B (Carbonaceous Grainse)

Graphite grains B (carbonaceous grains e) were formed in the same manneras in Synthesis Example 1 except that carbon black as a raw material waschanged. With respect to carbon black as a raw material, “HTC #20” madeby Nippon Steel Chemical Carbon Co., Ltd. was used. This carbon black iscarbon black of the type called fine thermal black (FT) in which theaverage primary grain size is 82 nm, the DBP absorption (x) is 29 ml/100g and the DBP absorption (y) of the compressed sample is 30 ml/100 g,and it is carbonaceous grains d used in this Example. When the resultinggrains were subjected to the X-ray diffraction measurement, a peakascribable to a graphite structure was observed, and it was found thatgraphite grains were formed. Lattice spacing calculated from adiffraction line corresponding to 002 spacing of graphite was 3.42 Å.The average primary grain size of the grains was 70 nm, the DBPabsorption (x) thereof was 28 ml/100 g, and the DBP absorption (y) ofthe compressed sample thereof was 28 ml/100 g.

SYNTHESIS EXAMPLE 3 Synthesis of Graphite Grains C (Carbonaceous Grainsc)

Carbon black “Niteron #10 Kai” and a boron powder were mixed such that amolar ratio of a carbon element to a boron element was 10:4, and themixture was charged into a silica crucible. A graphite sheet was put onthe upper surface of the crucible, and an electrode was connected withboth terminals thereof. An electric current was passed through theelectrode to generate heat in the graphite sheet and ignite the mixture,and graphite grains C (carbonaceous grains c) were obtained by aself-burning synthesis method using reaction heat generated in theformation of a carbide. When the resulting grains were subjected to theX-ray diffraction measurement, a peak ascribable to a graphite structurewas observed, and it was found that graphite grains were formed. Latticespacing calculated from a diffraction line corresponding to 002 spacingof graphite was 3.38 Å. Further, a peak with 2θ=37.8° ascribable to a021 diffraction line of B₄C was also identified. The X-ray diffractionchart is shown in FIG. 2. The average primary grain size of the grainswas 40 nm, the DBP absorption (x) thereof was 120 ml/100 g, and the DBPabsorption (y) of the compressed sample thereof was 86 ml/100 g.

SYNTHESIS EXAMPLE 4 Synthesis of Graphite Grains D

Graphite grains D were obtained in the same manner as in SynthesisExample 3 except that carbon black “HTC #20” and a titanium powder weremixed such that a molar ratio of a carbon element to a titanium elementwas 10:1. When the resulting grains were subjected to the X-raydiffraction measurement, a peak ascribable to a graphite structure wasobserved, and it was found that graphite grains were formed. Latticespacing calculated from a diffraction line corresponding to 002 spacingof graphite was 3.44 Å. Further, a peak with 2θ=41.5° ascribable to a200 diffraction line of TiC was also identified. The average primarygrain size of the grains was 71 nm.

SYNTHESIS EXAMPLE 5 Synthesis of Graphite Grains E (Carbonaceous Grainsf)

Graphite grains E (carbonaceous grains f) were obtained in the samemanner as in Synthesis Example 3 except that carbon black “HTC #20”, analuminum powder and a titanium oxide powder were mixed such that a molarratio of a carbon element to an aluminum element to a titanium elementwas 10:4:3. When the resulting grains were subjected to the X-raydiffraction measurement, a peak ascribable to a graphite structure wasobserved, and it was found that graphite grains were formed. Latticespacing calculated from a diffraction line corresponding to 002 spacingof graphite was 3.42 Å. Further, a peak with 2θ=43.4° ascribable to a113 diffraction line of Al₂O₃ and a peak with 2θ=41.5° ascribable to a200 diffraction line of TiC were also identified. The average primarygrain size of the grains was 70 nm, the DBP absorption (x) thereof was30 ml/100 g, and the DBP absorption (y) of the compressed sample thereofwas 29 ml/100 g.

SYNTHESIS EXAMPLE 6 Synthesis of Graphite Grains F

Graphite grains F were obtained in the same manner as in SynthesisExample 3 except that carbon black “HTC #20” and trimethoxyborane weremixed such that a molar ratio of a carbon element to a boron element was10:1. When the resulting grains were subjected to the X-ray diffractionmeasurement, a peak ascribable to a graphite structure was observed, andit was found that graphite grains were formed. Lattice spacingcalculated from a diffraction line corresponding to 002 spacing ofgraphite was 3.41 Å. Further, a peak with 2θ=37.8° ascribable to a 021diffraction line of B₄C was also identified. The average primary grainsize of the grains was 72 nm.

SYNTHESIS EXAMPLE 7 Synthesis of Graphite Grains G

The graphite grains C obtained in Synthesis Example 3 were charged intoa stainless steel tube, and a hot gas obtained by burning a mixture ofpropane and oxygen at a volume ratio of 1:8 was introduced therein. Thetemperature of the hot gas was 1,000° C., and the retention time was 5seconds. Water was then sprayed to cool the grains to 250° C., and theresulting graphite grains G were trapped with a bag filter. Latticespacing calculated from a diffraction line corresponding to 002 spacingof graphite was 3.40 Å. Further, a peak with 2θ=32.1° ascribable to a102 diffraction line of B₂O₃ was also identified. The average primarygrain size of the grains was 42 nm.

With respect to the graphite grains A to G obtained in SynthesisExamples 1 to 7, the raw materials, the resulting compound and theaverage primary grain size were all shown in Table 1-1.

TABLE 1-1 Synthesis Synthesis Synthesis Synthesis Synthesis SynthesisSynthesis Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Raw materials*1) FET (Niteron #10 Kai) 10 10 10 FT (HTC #20)10 10 10 10 boron powder  4  4 titanium powder  1 aluminum powder  4titanium oxide  3 trimethoxyborane  1 Resulting graphite grains A B C DE F G Resulting mineral C C C C C C C B₄C TiC Al₂O₃ B₄C B₂O₃ TiC Averageprimary grain size (nm) 38 70 40 71 70 72 42 *1)The figure is a mixingmolar ratio of a raw element.

EXAMPLE 1-1

100 parts by weight of electro-fused magnesia having a purity of 98%with a grain size adjusted, 2 parts by weight of the graphite grains Aobtained in Synthesis Example 1 and 3 parts by weight of a phenol resin(obtained by adding a curing agent to a novolak-type phenol resin) weremixed, and kneaded with a kneader. After the mixture was molded with afriction press, the molded product was baked at 250° C. for 8 hours.Consequently, after the heat treatment at 1,400° C., the apparentporosity was 9.2%, and the bulk specific gravity was 3.10. Further,after the heat treatment at 1,000° C., the dynamic elastic modulus was10.8 GPa, and after the heat treatment at 1,400° C., the dynamic elasticmodulus was 12.4 GPa. Moreover, the thickness of the decarbonized layerwas 7.8 mm, and the wear size was 11.0 mm.

EXAMPLES 1-2 TO 1-11, AND COMPARATIVE EXAMPLES 1-1 TO 1-7

Refractories were produced in the same manner as in Example 1-1 exceptthat the mixing raw materials were changed as shown in Tables 1-2 and1-3, and they were evaluated. The results are all shown in Tables 1-2and 1-3.

TABLE 1-2 Ex. 1-1 Ex. 1-2 Ex. 1-3 Ex. 1-4 Ex. 1-5 Ex. 1-6 Ex. 1-7 Ex.1-8 Mixing raw materials*1) magnesia 100 100 100 100 100 100 100 100graphite A 2 graphite B 2 graphite C 2 1 graphite D 2 7 12 2 FET(Niteron #10 Kai) 1 FT (HTC #20) 1 flake graphite phenol resin 3 3 3 3 33 3 3 Apparent porosity (%) 9.2 8.8 8.8 8.9 9.3 9.6 8.8 9.0 after 1,400°C. heat treatment Bulk specific gravity 3.10 3.13 3.12 3.12 3.07 3.013.11 3.10 after 1,400° C. heat treatment Dynamic elastic modulus (Gpa)10.8 17.1 12.0 17.8 6.7 5.1 16.4 10.1 after 1,000° C. heat treatmentDynamic elastic modulus (Gpa) 12.4 19.9 14.6 19.0 7.3 5.4 18.0 11.2after 1,400° C. heat treatment Thickness of decarbonized layer (mm) 7.86.1 5.2 4.9 7.1 8.2 5.7 5.4 Wear size (mm) 11.0 10.4 9.5 8.8 12.7 13.510.0 9.3 *1)The Mixing ratio is a weight ratio.

TABLE 1-3 CEx. CEx. CEx. Ex. 1-9 Ex. 1-10 Ex. 1-11 CEx. 1-1 CEx. 1-2CEx. 1-3 CEx. 1-4 1-5 1-6 1-7 Mixing raw materials*1) magnesia 100 100100 100 100 100 100 100 100 100 graphite E 2 graphite F 2 graphite G 2FET (Niteron #10 Kai) 2 FT (HTC #20) 2 7 flake graphite 5 20 expandedgraphite 5 phenol resin 3 3 3 3 3 3 3 3 3 3 Apparent porosity (%) 8.78.9 9.1 10.1 8.7 12.6 9.2 10.2 12.4 9.2 after 1,400° C. heat treatmentBulk specific gravity 3.12 3.11 3.11 3.06 3.12 2.96 3.06 2.98 2.99 3.18after 1,400° C. heat treatment Dynamic elastic modulus (Gpa) after 18.017.4 20.1 7.8 17.4 4.0 28.6 16.5 22.6 30.4 1,000° C. heat treatmentDynamic elastic modulus (Gpa) after 18.7 19.5 21.3 9.1 19.2 3.7 27.115.8 20.9 34.7 1,400° C. heat treatment Thickness of decarbonized layer(mm) 4.7 4.5 4.2 9.7 8.0 13.7 10.9 11.9 11.2 11.8 Wear size (mm) 9.3 8.98.4 13.2 11.1 24.0 17.8 21.1 19.0 18.4 *1)The Mixing ratio is a weightratio.

In case of using graphitized carbon blacks shown in Examples 1-1 and1-2, in comparison to the case of containing 5 parts by weight of flakegraphite or expanded graphite as shown in Comparative Example 1-4 or1-6, the dynamic elastic modulus is low, the excellent thermal shockresistance is obtained with the less carbon content, the thickness ofthe decarbonized layer and the wear size are also small, and theexcellent oxidation resistance and corrosion resistance are shown. Thesame thermal shock resistance as in containing 20 parts by weight offlake graphite as shown in Comparative Example 1-5 is attained by theaddition of as small as 2 parts by weight.

Further, these Examples show the small thickness of the decarbonizedlayer, the small wear size, the excellent oxidation resistance and theexcellent corrosion resistance in comparison to the case of usingungraphitized carbon blacks shown in Comparative Examples 1-1 and 1-2.

These facts prove the superiority of using the quite fine grains in thenanometer order and the superiority of using the graphitized grains.

Still further, in Examples 1-3, 1-4, 1-9 and 1-10 using the graphitegrains containing boron, titanium or aluminum, in comparison to Examples1-1 and 1-2 using the graphite grains free from these elements, it isfound that the thickness of decarbonized layer and the wear size aresmaller, and the oxidation resistance and the corrosion resistance aremore improved.

Furthermore, in case of using the graphite grains containing the boronelement and oxidized as shown in Example 1-11, in comparison to Example1-3 using the graphite grains before the oxidation, the oxidationresistance and the corrosion resistance are improved.

Next, with respect to the carbonaceous grains b, c, e and f obtained inSynthesis Examples 1, 3, 2 and 5 and the carbonaceous grains a and dwhich are carbon blacks as raw materials used in these SynthesisExamples, the raw materials, the treatment method, the DBP absorption(x), the DBP absorption (y) of the compressed sample, the ratio (x/y)and the average primary grain size were all shown in Table 2-1.

TABLE 2-1 Carbonaceous grains (A) Carbonaceous grains (B) CarbonaceousCarbonaceous Carbonaceous Carbonaceous Carbonaceous Carbonaceous grainsa grains b grains c grains d grains e grains f Raw materials*1) FET(Niteron #10 Kai) 10 10 10 FT (HTC #20) 10 10 10 boron powder 4 aluminumpowder 4 titanium oxide 3 Carbon black treatment method untreatedSynthesis Synthesis untreated Synthesis Synthesis Example 1 Example 2Example 3 Example 4 DBP absorption (x) (ml/100 g) 126 118 120 29 28 30DBP (y) of compressed sample 89 85 86 30 28 29 (ml/100 g) Ratio (x/y)1.42 1.39 1.4 0.97 1 1.03 Average primary grain size (nm) 41 38 40 82 7070 *1)The figure is a mixing molar ratio of a raw element.

EXAMPLE 2-1

100 parts by weight of electro-fused magnesia having a purity of 98%with a grain size adjusted as a refractory filler, 0.5 part by weight ofcarbon black “Niteron #10 Kai” (carbonaceous grains a) as carbonaceousgrains (A), 1.5 parts by weight of carbon black “HTC #20” (carbonaceousgrains d) as carbonaceous grains (B) and 3 parts by weight of a phenolresin (obtained by adding a curing agent to a novolak-type phenol resincontaining a solvent) were mixed, and kneaded with a kneader. After themixture was molded with a friction press, the molded product was bakedat 250° C. for 8 hours to obtain a refractory. The resulting refractorywas evaluated. Consequently, after the heat treatment at 1,400° C., theapparent porosity was 8.8%, and the bulk specific gravity was 3.10.Further, after the heat treatment at 1,000° C., the dynamic elasticmodulus was 11.3 GPa. After the heat treatment at 1,400° C., the dynamicelastic modulus was 12.7 GPa. The thickness of the decarbonized layerwas 7.7 mm, and the wear size was 10.8 mm.

EXAMPLES 2-2 TO 2-10, AND COMPARATIVE EXAMPLES 2-1 TO 2-5

Refractories were produced in the same manner as in Example 2-1 exceptthat the mixing raw materials were changed as shown in Tables 2-2 and2-3, and they were evaluated. The results are all shown in Tables 2-2and 2-3.

EXAMPLE 2-11

The test was conducted using the same amounts of the same raw materialsas in Example 2-2 and changing the mixing method only. First, 0.2 partby weight of carbon black “Niteron #10 Kai” (carbonaceous grains a) and3 parts by weight of the same phenol resin as used in Example 2-1 werecharged into a universal mixing stirrer manufactured by DaltonCorporation, and mixed. The resulting mixture was mixed with 1.8 partsby weight of carbon black “HTC #20” (carbonaceous grains d) and 100parts by weight of the same fused magnesia as used in Example 2-1, andkneaded with a kneader. After the mixture was molded with a frictionpress, the molded product was baked at 250° C. for 8 hours to obtain arefractory. The resulting refractory was evaluated, and the results areall shown in Tables 2-2 and 2-3.

TABLE 2-2 Ex. 2-1 Ex. 2-2 Ex. 2-3 Ex. 2-4 Ex. 2-5 Ex. 2-6 Ex. 2-7 Ex.2-8 Mixing raw materials (weight ratio) magnesia 100 100 100 100 100 100100 100 carbonaceous grains a 0.5 0.2 1 1.8 carbonaceous grains b 0.50.5 carbonaceous grains c 0.5 0.5 carbonaceous grains d 1.5 1.8 1 0.21.5 1.5 carbonaceous grains e 1.5 1.5 phenol resin 3 3 3 3 3 3 3 3Apparent porosity (%) 8.8 8.7 9.5 10.4 8.7 8.4 8.7 8.6 after 1,400° C.heat treatment Bulk specific gravity 3.10 3.11 3.06 3.05 3.10 3.11 3.113.12 after 1,400° C. heat treatment Dynamic elastic modulus (GPa) 11.313.6 10.1 8.0 11.5 11.8 11.6 11.7 after 1,000° C. heat treatment Dynamicelastic modulus (GPa) 12.7 15.7 10.5 8.9 12.9 13.0 13.2 13.5 after1,400° C. heat treatment Thickness (mm) of decarbonized layer 7.7 7.97.9 8.5 7.4 6.5 6.9 5.5 Wear size (mm) 10.8 10.8 10.9 12.1 10.4 9.2 10.28.3

TABLE 2-3 Ex. 2-9 Ex. 2-10 Ex. 2-11 CEx. 2-1 CEx. 2-2 CEx. 2-3 CEx. 2-4CEx. 2-5 Mixing raw materials (weight ratio) magnesia 100 100 100 100100 100 100 100 carbonaceous grains a 4 0.2 2 carbonaceous grains c 0.5carbonaceous grains d 8 1.8 2 carbonaceous grains f 1.5 flake graphite 5expanded graphite 5 phenol resin 3 3 3 3 3 3 3 3 Apparent porosity (%)8.5 12.8 8.5 10.1 8.7 9.2 9.2 12.4 after 1,400° C. heat treatment Bulkspecific gravity 3.13 2.97 3.11 3.06 3.12 3.18 3.06 2.99 after 1,400° C.heat treatment Dynamic elastic modulus (GPa) 11.9 2.2 13.2 7.8 17.4 30.428.6 22.6 after 1,000° C. heat treatment Dynamic elastic modulus (GPa)13.8 2.3 15.1 9.1 19.2 34.7 27.1 20.9 after 1,400° C. heat treatmentThickness (mm) of decarbonized layer 4.6 13.9 7.6 9.7 8.0 11.8 10.9 11.2Wear size (mm) 7.4 20.2 10.5 13.2 11.1 18.4 17.8 19.0

In Examples 2-1 to 2-4, carbon black [carbonaceous grains (A)] havingthe DBP absorption of 80 ml/100 g or more and carbon black [carbonaceousgrains(B)] having the DBP absorption of less than 80 ml/100 g are bothused, and mixed. Consequently, the low dynamic elastic modulus is shownto provide the good thermal shock resistance. Further, the thickness ofthe decarbonized layer is also small to show the good oxidationresistance. The wear size is also small to show the good wearresistance. On the contrary, in Comparative Example 2-1 using onlycarbon black [carbonaceous grains (A)] having the DBP absorption of 80ml/100 g or more, the thickness of the decarbonized layer is great toshow the poor oxidation resistance. The wear size is also large to showthe poor wear resistance. In Comparative Example 2-2 using only carbonblack [carbonaceous grains (B)] having the DBP absorption of less than80 ml/100 g, the dynamic elastic modulus is high to show the poorthermal shock resistance.

As the carbonaceous grains (A) are added at a ratio of 10/100 (Example2-2), 25/100 (Example 2-1) or 50/100 (Example 2-3) based on the totalamount of the carbonaceous grains (A) and (B), the dynamic elasticmodulus is much decreased in comparison to Comparative Example 2-2 usingonly the carbonaceous grains (B). For example, in Example 2-2, with theaddition of the carbonaceous grains (A) at a low ratio of 10/100, thedynamic elastic modulus is greatly improved from 17.4 GPa to 13.6 GPaafter the heat treatment at 1,000° C., and from 19.2 GPa to 15.7 GPaafter the heat treatment at 1,400° C. At this time, in these Examples2-1 to 2-3, both of the thickness of the carbonized layer and the wearsize show rather smaller values than in Comparative Example 2-2 usingonly the carbonaceous grains (B). Not only is the thermal shockresistance greatly improved, but also the oxidation resistance and thewear resistance are excellent.

Meanwhile, in Example 2-4 using further the carbonaceous grains (B) at aratio of 10/100 based on the total amount of (A) and (B), the dynamicelastic modulus is greatly improved in comparison to Comparative Example2-1 using only the carbonaceous grains (A), while the oxidationresistance and the wear resistance are approximately the same as inComparative Example 2-1. That is, in comparison to the use of thecarbonaceous grains (B) only, the additional use of the small amount ofthe carbonaceous grains (A) can provide the refractory excellent inthermal shock resistance, oxidation resistance and wear resistance.

In Comparative Example 2-3 in which the carbonaceous grains are notincorporated, the thermal shock resistance is notably deteriorated, andthe oxidation resistance and the corrosion resistance are also poor.Further, in case of using flake graphite (Comparative Example 2-4) orexpanded graphite (Comparative Example 2-5) as a carbonaceous rawmaterial, the thermal shock resistance is worse than in Examples 2-1 to2-4 even though this graphite is used in an amount of 5 parts by weightwhich is larger than 2 parts by weight, the use amount of carbon blackin Examples 2-1 to 2-4. At this time, both of oxidation resistance andwear resistance are worse than in Examples 2-1 to 2-4.

In case the graphite grains obtained by graphitizing carbon black areused as the carbonaceous grains (A) (Example 2-5), the oxidationresistance and the corrosion resistance are improved in comparison tothe case of using carbon black as both of the carbonaceous grains (A)and (B) (Example 2-1). Further, in case the graphite grains obtained bygraphitizing carbon black and containing at least one element selectedfrom metals, boron and silicon (Example 2-7) are used as thecarbonaceous grains (A), the oxidation resistance and the corrosionresistance are more improved.

In case the graphite grains obtained by graphitizing carbon black areused as both of the carbonaceous grains (A) and (B), the oxidationresistance and the corrosion resistance are more improved than inExample 2-5. Besides, in case the carbonaceous grains (A) are thegraphite grains obtained by graphitizing carbon black and containing atleast one element selected from metals, boron and silicon (Example 2-8),the oxidation resistance and the corrosion resistance are much moreimproved. In case the graphite grains obtained by graphitizing carbonblack and containing at least one element selected from metals, boronand silicon are used as both of the carbonaceous grains (A) and (B), thebest results of the oxidation resistance and the corrosion resistanceare provided.

In addition, in Example 2-11, the mixture of the same raw materials atthe same mixing ratio as in Example 2-2 is used, and the carbonaceousgrains (A) are previously dispersed in the organic binder and then mixedwith the other raw materials. Consequently, the dispersibility of thecarbonaceous grains (A) in the matrix is improved, with the result thatthe thermal shock resistance, the oxidation resistance and the corrosionresistance are improved in comparison to Example 2-2 in which all of theraw materials are mixed at the same time.

INDUSTRIAL APPLICABILITY

As has been thus far described, the invention can provide therefractories excellent in corrosion resistance, oxidation resistance andthermal shock resistance, especially the carbon-contained refractorieshaving the low carbon content. Such carbon-contained refractories havingthe low carbon content are useful because they exhibit less carbonpickup in a molten steel and cause less heat dissipation fromcontainers. Further, the invention can also provide the refractory rawmaterials for obtaining such refractories.

The invention claimed is:
 1. A refractory which is obtained by molding acomposition comprising a refractory filler and graphite grains, whereinthe graphite grains comprise at least one element selected from thegroup consisting of metals, boron and silicon, and have an average grainsize of 500 nm or less.
 2. The refractory as claimed in claim 1, whereinthe composition comprises 100 parts by weight of the refractory fillerand from 0.1 to 10 parts by weight of the graphite grains.
 3. Therefractory as claimed in claim 1, wherein the refractory fillercomprises magnesia.
 4. The refractory as claimed in claim 1, wherein thegraphite grains have an average grain size of 100 nm or less.
 5. Therefractory as claimed in claim 1, wherein the graphite grains have a DBPabsorption (x) of 80 ml/100 g or more.
 6. The refractory as claimed inclaim 1, wherein a ratio (x/y) of a DBP absorption (x) of the graphitegrains to a DBP absorption (y) of a compressed sample of the graphitegrains is 1.15 or more.
 7. The refractory as claimed in claim 1, whereinthe composition further comprises carbon black having a DBP absorption(x) of less than 80 ml/100 g.
 8. A refractory which is obtained bymolding a composition comprising a refractory filler and graphitegrains, wherein the graphite grains comprise at least one elementselected from the group consisting of metals, boron and silicon, and areobtained by graphitizing carbon black.
 9. The refractory as claimed inclaim 8, wherein the composition comprises 100 parts by weight of therefractory filler and from 0.1 to 10 parts by weight of the graphitegrains.
 10. The refractory as claimed in claim 8, wherein the refractoryfiller comprises magnesia.
 11. The refractory as claimed in claim 8,wherein the graphite grains have an average grain size of 100 nm orless.
 12. The refractory as claimed in claim 8, wherein the graphitegrains have a DBP absorption (x) of 80 ml/100 g or more.
 13. Therefractory as claimed in claim 8, wherein the ratio (x/y) of a DBPabsorption (x) of the graphite grains to a DSP absorption (y) of acompressed sample of the graphite grains is 1.15 or more.
 14. Therefractory as claimed in claim 8, wherein the composition furthercomprises carbon black having a DBP absorption (x) of less than 80ml/100 g.